This invention relates to a process for the separation of C.sub.3+ hydrocarbons from a gas stream under super-atmospheric pressure, said gas stream being cooled, partially condensed and separated into a liquid and a gaseous fraction and wherein the gaseous fraction, is engine expanded (expansion while performing external work--also called work expansion) and the liquid fraction is fractionated by rectification into a product stream containing mostly C.sub.3+ hydrocarbons and a residual gas stream containing mostly lower boiling components.
Such processes are used, for example, in obtaining an LPG fraction (C.sub.3 /C.sub.4 hydrocarbons) from natural gases or refinery gases, or in separating propane and higher hydrocarbons (optionally unsaturated hydrocarbons as well) from refinery or cracked gases. A process of this kind is described in an earlier German patent application No. P 34 08 760.5 filed Mar. 9, 1984 in Germany having a common assignee and corresponding substantially to U.S. application Ser. No. 709,742 filed Mar. 8, 1985 by Bauer et al, said U.S. application being incorporated by reference herein. An important feature of this earlier application is that the cold obtained during engine expansion of the gaseous fraction remaining after partial condensation is used not for production of reflux liquid in the rectification column, but instead for the cooling and partial condensation of the crude gas. Therefore, in this process it is unnecessary to feed the light components of the gas stream into the rectification column. The elimination of the introduction of the light components of the feedstock stream (especially C.sub.1 and C.sub.2 hydrocarbons) into the column makes it possible to perform the rectification at a higher temperature. Thus, the possibility of using a simple and inexpensive external cold cycle for cooling the overhead from the rectification column constitutes a considerable improvement in conducting the process. Nevertheless a further improvement is still desirable, especially to increase the yield of separated C.sub.3+ hydrocarbons without having to pass entire feed stream into the rectification column, or without having to perform the partial condensation at a lower temperature. In the process of the above described application, a particular disadvantage is the substantial compressor power required for recompression of the light fractions in those cases where the C.sub.2- fraction is to be delivered under high pressure (e.g., into a natural gas pipeline).